Various types of vehicles for use as both land and air transportation have been proposed. Generally, such vehicles have taken the form of an automobile-type passenger body and wheeled chassis, in combination with a separable wing and rudder/elevator tail section. In a typical configuration, single or dual pusher type propellers are mounted rearwardly of the passenger body and are driven by one or more internal combustion engines.
Compromises must be made in the construction and configuration of such a land-air vehicle. The vehicle should not be excessively wide for land vehicle use, but must have an aerodynamic shape for good flying characteristics and fuel economy. Wings should be designed for low-speed take-offs and landings while providing good flight characteristics at cruising speeds. However, most vehicles of the type described have been structurally unsound and aerodynamically inefficient.
Several disadvantages of conventional single or dual type propellers in aircraft or land-air vehicles have been recognized. Conventional propellers are designed for maximum thrust and acceleration at take-off speeds and below to ensure short, safe take-off rolls. Such propellers have poor thrust capabilities at other than take-off speeds since as airspeed increases, the proportion of the propeller that creates drag increases, but the propeller thrust area decreases.
Conventional propeller driven aircraft utilize air cooled engines which produce high torque at operating speeds of around 2500-3000 rpm. The propeller is directly driven by the engine. Fuel efficiency at such low engine/propeller speeds is not very good since the aircraft will travel relatively slowly. However, increasing the engine speed to propel the aircraft at higher speeds and increase fuel economy causes other problems. The tip speed of a larger diameter propeller (approximately 80") being driven at high speed can approach the speed of sound. Shock waves will form at the tip, creating additional drag and possible propeller damage. Vortices formed at the tips of the propellers create drag and increase the noise level of the aircraft. In addition, some form of variable pitch should be provided, since optimum propeller pitch angle changes as the speed of the aircraft increases. Because of these problems, conventional propeller driven general aviation aircraft are limited to top airspeeds of around 150 to 200 knots.
To increase cruise efficiency and overcome the disadvantages of the usual two-bladed propeller, smaller multi-bladed propellers called cruise propellers have been used. However, multi-bladed propellers have the disadvantage of more tip vortices, which significantly increase the propeller drag. Because these propellers are designed to be more efficient at cruise speeds, they create longer take-off rolls.
To reduce or eliminate tip vortices in propeller type aircraft, a shroud has been used around the propeller. Such a configuration is commonly known as a "ducted fan". Ducted fans usually include four or more blades, generally of smaller diameter than those of conventional propellers and have thin, narrow airfoils. Ducted fans tend to be quieter and more efficient than conventional propellers because tip vortices are eliminated. The shroud around the fan also lends a measure of safety to ground personnel who are near the aircraft.
A ducted fan can be designed for either low speed or high speed operation, but not for both. A low speed ducted fan (e.g. less than approximately 70 knots air speed) outperforms a conventional propeller at take-off speeds while a high speed (e.g. greater than approximately 70 knots air speed) ducted fan theoretically outperforms a conventional propeller at cruising speeds. When accelerating from low speeds to high speed flight, the blade area of both conventional propellers and ducted fans must change for efficient operation. In the case of conventional propellers, a variable pitch device is used to open the blade angle of the propeller into an airstream as it accelerates from low speed to high speed. Variable pitch provides adequate propeller efficiency at cruise speeds while producing sufficient thrust at take-off speeds. The ducted fan design, however, does not work well with variable pitch. When accelerating from low speeds to high speed flight, the blade area of a ducted fan must change over a greater range than conventional propellers for efficient operation. Hence, either the number or the size of the blades must change, obvious impossibilities. Thus, there is presently available no one particular ducted fan design which produces high thrust at both low and high speeds, while maintaining good fuel economy.
To increase the thrust of a low-speed ducted fan it is known to form the shroud having a cross-section in the form of a cambered airfoil. While this technique increases the low-speed thrust slightly, the additional thrust is gained at the expense of high speed efficiency since the shroud creates drag at high speed. Shrouds for high speed ducted fans have a symmetric airfoil shape developing no additional thrust, but also minimizing drag from the shroud.
If a conventional air-cooled aircraft engine is used to drive a ducted fan, some sort of gearing must be provided since the conventional engine develops maximum torque and power in the range of 2500-3000 rpm while a high speed ducted fan is most efficient and produces maximum thrust at speeds in excess of 6000 rpm. The gearing arrangement introduces a power loss of about 5% in the drive train and lowers the overall performance of the aircraft.
It is well known that single propeller aircraft, both conventional and ducted fan types, have asymmetrical flight characteristics due to torque and "cork-screw" effect exerted by the single rotating propeller on the aircraft body. In some multi-propeller aircraft, opposing pairs of propellers rotate in opposite directions to provide symmetrical flight characteristics and overcome the "cork-screw" effect. However, these designs usually employ two separate engines to drive the propellers with a concomitant increase in aircraft weight, and increased fuel consumption which results in a reduction of fuel economy.
It is therefore an object of the invention to provide an aircraft having good fuel economy at high cruising speeds and which is relatively quiet in operation.
It is a further object to provide such an aircraft having symmetrical flight characteristics and good maneuverability at both low and high speeds.
It is an additional object to provide an aircraft having tail and wing sections detachable from the passenger section so that the passenger section can be used as a land vehicle upon arrival at a destination.